1. Field of the Invention
The present invention relates to a solid state facilitated transport separation membrane for the separation of alkenes prepared by using a polymer electrolyte, wherein the membrane has high permeance and selectivity of alkene-series hydrocarbons even under dry conditions. More specifically, the present invention relates to a solid state facilitated transport separation membrane prepared by coating a polymer electrolyte layer consisting of a metal salt and a polymer onto a porous supported membrane, wherein the separation membrane is characterized in that its permeance and selectivity to alkenes is high and in that the complex formed by a polymer ligand in the polymer electrolyte maintains its activity as a carrier for alkene-series hydrocarbons for a long time, even under dry conditions.
2. Description of the Prior Art
Alkene-series hydrocarbons, such as ethylene and propylene, are important raw materials that form the basis of the current petrochemical industry. They are primarily produced by high temperature thermal decomposition of naphtha that is obtained from a purification process of petroleum. However, since alkane-series hydrocarbons such as ethane and propane are also produced by this process of thermal decomposition, use of alkene-series hydrocarbons as raw materials should be preceded by the separation process of mixtures having alkene-series hydrocarbons. Currently, a sub-zero distillation process is generally used in the separation of these mixtures. For example, a column having at least 100 trays of theoretical number at a low temperature of xe2x88x9225xc2x0 C. and at a high pressure of about 23 atm is required in the sub-zero distillation process to separate the mixture of ethylene/ethane. In the separation of propylene/propane, similar operation conditions are also required. Therefore, there has been a continued need for the development of a new separation process that can replace the prior distillation process, which requires investment of large-scale equipment and high-energy costs.
A separation process that could be considered as a replacement for distillation processes is one that uses a separation membrane. The separation membrane process has progressed remarkably over the past few decades in the field of separating nitrogen/oxygen, nitrogen/carbon dioxide and nitrogen/methane, and the like. In the separation of mixtures, such as alkenes/alkanes, however, a satisfactory separation performance cannot be accomplished by using general polymer separation membranes because they are similar in both molecular size and in such physical properties as the degree of condensation.
The process utilizing the separation membrane that uses the concept of facilitated transport could be considered as an alternative process. Facilitated transport refers to a transport phenomenon in which the general mass transport according to Fick""s law and the mass transport mediated by a carrier occur simultaneously. A supported liquid membrane is one membrane prepared by using such a concept. This membrane facilitates said mass transport by filling a porous membrane with a solution containing a carrier. Supported liquid membranes of this type have succeeded to a certain extent but have drawbacks in that solvent is gradually lost and permeance is gradually reduced over time. By using a membrane of this type, for example, Steigelmann and Hughes prepared a supported liquid membrane wherein the selectivity of ethylene/ethane is about 400 to 700 and the permeance of ethylene is 60 GPU [1 GPU=1xc3x9710xe2x88x926 cm3(STP)/cm2 sec cmHg] (see, U.S. Pat. Nos. 3,758,603 and 3,758,605). These values show considerably good results, but there remains the problem that such a state cannot be maintained for an extended period of time due to the aforementioned solvent loss.
To solve this problem of the supported liquid membrane, a method which enables facilitated transport by substituting a suitable ion in an ion exchange resin was proposed by Kimura, et al. (see U.S. Pat. No. 4,318,714). However, this ion exchange resin membrane also has a drawback in that the facilitated transport phenomenon is exhibited only under humid conditions, which is similar to the supported liquid membrane. This membrane, which must always be maintained in humid conditions, is inconvenient and inefficient in the actual separation process. Also, a carrier loss into the aqueous phase can still occur, as with the supported liquid membrane.
Another method proposed by Ho is a process for producing a complex by using water-soluble glassy polymers, such as polyvinylalcohol (see U.S. Pat. Nos. 5,015,268 and 5,062,866). However, this method also had a drawback in that satisfactory results can be obtained only where feed gas is saturated with water vapor by passing it through water, or where the membrane is swelled by using ethylene glycol or water.
Kraus, et al. also have developed a facilitated transport separation membrane by using a similar method (see U.S. Pat. No. 4,614,524). According to this patent, an Ag ion is substituted for an ion exchange membrane, such as Nafion, and said membrane is then plasticized, for example, with glycerol. However, this membrane could not be utilized because the selectivity of ethylene/ethane is as low as about 10 when dry feed was used. And when a plasticizer was not used, the selectivity could not be seen.
Thus, the development of a solid state facilitated transport separation membrane in which selectivity and permeance are high and stable even under dry operating conditions is sincerely required in the separation of alkene/alkane hydrocarbons.
Considering that the usual polymer separation membrane cannot separate alkene/alkane mixtures having similar molecular size or physical properties, as described above, use of a facilitated transport separation membrane capable of selectively separating only alkene hydrocarbons is needed. The conventional facilitated transport separation membranes, however, have a disadvantage in that the mobility of a carrier should be increased by using the following method: filling a solution containing a carrier into a porous membrane, adding a plasticizer or saturating feed gases with water vapor, etc. Such a conventional facilitated transport separation membrane cannot be utilized due to the problem of declining stability of the membrane, since the materials added are gradually lost with time. There is also the problem of having to remove water that is contained in the feed before the preparation of the product.
Therefore, the purpose of the present invention is to prepare a facilitated transport separation membrane by introducing, for example, the polymer electrolytes used in polymer batteries into the facilitated transport separation membrane, wherein the problem of stability, such as with carrier loss, is solved and unsaturated hydrocarbons such as alkene can be separated even under dry conditions.
That is, an object of the present invention is to prepare a solid state facilitated transport separation membrane for use in the separation of alkene-series hydrocarbons from alkenes/alkanes mixtures, wherein the separation membrane has high permeance and selectivity for alkene-series hydrocarbons under dry operating conditions.
This object of the present invention is accomplished through the use of a solid state facilitated transport composite separation membrane; said membrane consisting of a porous supported membrane and a polymer electrolyte layer consisting of a metal salt and a polymer, wherein the composite separation membrane has an ideal selectivity expressed by [permeance of pure alkenes/permeance of pure alkanes] of at least 100.
The present invention is described in greater detail below.
The solid state facilitated transport separation membrane according to the present invention is composed of a polymer electrolyte having a selective permeance for alkene-series hydrocarbons and a porous supported membrane supporting it.
Any supported membrane having good permeance and sufficient mechanical strength can be used in the present invention. For example, both a conventional porous polymer membrane and a ceramic membrane may be used. Plate type, tubular type, hollow fiber type, capillary type or other types of supported membranes may also be used.
The polymer electrolyte used in the present invention is composed of a polymer and a metal salt. The metal salt in the electrolytes is not simply dispersed or mixed in the polymers, but is dissociated into a cation of metals and an anion of salts on the polymer. Contrary to conventional membranes, the solid state facilitated transport separation membrane of the present invention does not require the addition of water to maintain the activity of the carrier and other materials to swell the polymer matrix, and it selectively facilitates the transport of alkene feed under dry conditions.
That which substantially affects the selective separation of alkene-series hydrocarbons in the solid state facilitated transport separation membrane according to the present invention is a polymer electrolyte containing a carrier (a metal ion), wherein the permeation selectivity of alkene-series hydrocarbons from the corresponding alkane-series hydrocarbons is determined according to the properties of the electrolyte.
A metal salt used in the present invention is composed of a cation of metal and an anion of salt, and, therefore, they are dissociated with ions on the polymer. As a result, the metal ion directly takes part in the facilitated transport via reversible reaction with the double bond of alkene-series hydrocarbons. When alkene-series hydrocarbons are facilitated-transported in the polymer electrolyte, it is possible to interact the metal ion with the anions of salt, the electron donor in the polymer ligands and the electron of the alkene-series hydrocarbons. Accordingly, the polymer, the cation of metals, and the anion of salts must be properly selected to obtain a membrane with high selectivity and permeance.
The polymer used in the invention must easily form a complex with the metal salt as mentioned above to allow the reversible interaction of the metal cation with alkenes. A polymer having a heteroatom that can form a covalent bond with a metal ion may be considered as one such polymer.
In the present invention, the polymers containing a nitrogen atom or amide group in the repeat unit are preferred among the above polymers. Here, nitrogen or oxygen atoms coordinate with metal atoms to make complexes. These polymers are poly(ethylene imine), poly(alkyl oxazoline), poly(vinyl pyrrolidone), poly(acryl amide), poly(N-isopropyl acryl amide), etc., but the invention is not limited to the polymers described herein.
Next, the metal salts to be used as carriers are described. In the present invention, as mentioned above, the transport of alkenes is selectively facilitated, which results from the metal ion which has formed a complex with a polymer ligand in the polymer electrolyte reacting reversibly with alkene-series hydrocarbons.
As cations of the metal in the present invention, silver and copper, known as readily forming complexes with alkene-series hydrocarbons, are preferred, and tetrafluoroborate, trifluoromethane acetate, trifluoromethane sulfonate and hexafluorophosphate, etc. are preferred as anions of the metal salts. Further, metal salt, such as silver tetrafluoroborate (AgBF4), silver trifluoromethane sulfonate (AgCF3SO3), silver trifluoromethane acetate (AgCF3CO2) and silver hexafluorophosphate (AgPF6), are preferably used in the present invention as carriers which facilitate the transport of alkene-series hydrocarbons in the polymer electrolytes, but the invention is not limited to the metal salts illustrated herein.
The following describes a process for the preparation of the solid state facilitated transport separation membrane of the invention.
The solid state facilitated transport separation membrane of the present invention is prepared first by dissolving a transition metal salt and a polymer constituting a polymer electrolyte in a liquid solvent to form a coating solution, applying the solution onto a porous supported membrane and then drying it. The liquid solvent used in the process can be any that can dissolve the transition metal and polymer without impairing the supported membrane. If the polymer constituting the solid electrolytes is water-soluble, water is used as a solvent. The concentrations of metal salts and polymers in the coating solution are determined by considering the thickness of the polymer electrolytes solution formed directly after application and the thickness after drying.
For example, blade/knife coating, Mayer bar coating, dip coating, air knife coating, and the like can be used as the method by which the polymer electrolyte solution is applied to the supported membrane. The thickness of the polymer electrolyte formed on the supported membrane after drying is preferably as thin as possible to enhance permeance.
The solid state facilitated transport separation membrane according to the invention is suitable for application to the actual separation process for alkenes/alkanes because: the selectivity for alkene-series hydrocarbons is high, being at least 100 of the ideal selectivity expressed by [permeance of pure alkene/permeance of alkane]; the activity of a carrier is maintained even under completely dry operating conditions; and the long-term operation stability is high.
The examples below illustrate the present invention in detail, but the invention is not limited to the scope thereof.